XL Nickel Wound electric guitar strings are the standard against
  which all other strings are measured. Our best-belling electric
  set, XL Nickel has been revered by players everywhere since 1974.
  Made with a high carbon steel core and nickel-plated steel wrap
  wire, XL Nickel strings have a bright, versatile tone, ideal for
  a wide variety of musical styles. Popular with studio musicians
  for doubling 6-string guitar parts to achieve a 12-string effect,
  this 10-26 High Strung/Nashville Tuning gauge utilizes the higher
  'octave' strings of a 12-string set to achieve a shimmering,
  "high-strung" tone.